A conventional technology regarding a lithium-ion secondary battery proposes to improve clone efficiency, cycle characteristic and chargeable capacity through speedily raising a positive-electrode potential at a final stage of charging (for example, JP 2009-093924A). This lithium-ion secondary battery is charged through a constant-current and constant-voltage method or a multi-step constant-current method so that the positive-electrode potential speedily rises at the final stage of charging.
According to the constant-current and constant-voltage charging method or the multi-step constant-current charging method, the positive-electrode potential relative to a negative-electrode potential (for example, potential relative to a carbon negative electrode) rises with charging and discharging as well as aging deterioration. Further, when the positive-electrode potential rises quickly at the final stage of charging, electrolyte solution generates decomposition gas. In a case that a positive-electrode material (cathode active material and the like having an Olivine-type crystal structure), a resistance of which increases at the final stage of charging, the rising of the positive-electrode charge potential of the lithium metal reference causes an excessive voltage rapidly at the end of charging made at a constant C rate and also other problems such as deposition of lithium. Further it is likely to become a factor to promote more gas generation.